Electroplating gold and thiomalate electrolyte therefor



United States Patent 4 Claims ABSTRACT OF THE DISCLOSURE Bright goldelectrodeposits are obtained from an aqueous alkaline bath containinggold, added as alkali metal gold thiomalate, and a minute proportion ofa soluble arsenic compound.

This application is a continuation-in-part of U.S. application Ser. No.584,679 filed Oct. 6, 1966, of the same title, now abandoned.

This invention relates to a process and electrolyte for plating ofbright gold.

Among the objects of the invention is to provide an electrolyte andprocess for plating bright gold.

Among other objects of the invention is to provide a process andelectrolyte for plating bright 24 karat gold.

The objects of the invention are attained by dissolving a goldthiomalate complex in an alkaline solution to provide an electrolytecontaining a dissolved alkali metal gold thiomalate complex, adding aminute amount of arsenic to said bath (as a soluble salt) and adjustingthe pH to about to 12.

The bath is operated at a temperature of about 24- 82 C. and at acurrent density of 2-10 amperes/ftfi.

The arsenic is added in proportions of approximately 10 mg. to 0.12 gm.per liter of solution and can beadded in the form of any arseniccompound which is soluble in the bath at the said pH range of 10 to 12.Since the bath is alkaline, the alkali metal arsenites and the variousalkali metal arsenates are well suited as the addition compounds. Thearsenic can also be added to the bath in the form of the oxide or thehalide in which case an alkali metal arsenite or arsenate is formed withthe alkali of the bath.

Processes of electroplating metals including gold by passing an electriccurrent through a bath containing mercapto-metal complex salts aredisclosed in U.S. 3,238,112 and among the mercapto compounds which formsaid metal complexes, Z-mercaptosuccinic acid (thiomalic acid) isdisclosed. Without the arsenic addition as disclosed in the presentapplication, gold deposited from the alkali metal gold thiomalatecomplex is, at most, semi-bright when thicknesses of 100 ,u." areapproached. t"-microinches) Alkali metal gold thiomalates can be made bythe process described in U.S. 1,994,213 or by the process of U.S.3,238,112.

The preferred ranges of components and conditions for obtaining brightgold deposits are as follows:

Thiomalic acid40-100 g./l. (preferably 45-52 g./l.)

Alkali hydroxide-25-60 g./l.

Gold (as alkali metal gold thiomalate)-4-8 g./l.

As (as soluble compound)-10 mg. to 0.12 g./1. (preferably 10-30 mg./l.)

3,520,785 Patented July 14, 1970 pH-l0l2 Temperature-2482 C. Currentdensity2l0 amps/ft.

Bright gold deposits can be obtained with more than 30 mg./l. ofarsenic, but the additional arsenic does not increase the brightness.

The following example is given to illustrate, in detail, a preferredembodiment of the invention, but it will be understood that theinvention may be otherwise embodied within the scope of the broaderclaims.

EXAMPLE An aqueous solution of thiomalic acid in sufficient proportionsto provide 52 g. of the acid is mixed with an aqueous solution of sodiumhydroxide in sufficient proportions to provide 40 g. of NaOH.Approximately 8 g. of previously formed sodium gold thiomalate isdissolved in the resultant mix and water is added to make 1 liter.Approximately 35 mg. of sodium arsenite (or sufficient to provide about20 mg. of As) is added to the solution. The pH of the bath is measuredand adjusted to pH 11.5, if necessary, by the addition of NaOH orthiomalic acid. Products plated from the bath at a temperature of C. andcurrent density of 2 amps/ft. (voltage 0.7) are bright in thicknesses ashigh as 500a" or more.

We claim:

1. An aqueous alkaline electroplating bath for the deposition of brightgold consisting essentially of the following ingredients in theproportions set forth:

Thiomalic acid-40-l00 g./l.

Alkali hydroxide25-60 g./l.

Gold (as alkali metal gold thiomalate)-4-8 g./I.

Arsenic (as a soluble compound)10 mg. to 0.12 g./l.

2. The bath of claim 1 wherein the arsenic is added in the proportion of10-30 mg./l.

3. A process for the electroplating of bright gold comprisingelectrolyzing an aqueous solution containing the following ingredientsin the proportions set forth:

Thiomalic acid45-52 g./l.

Alkali hydroxide2540 g./l.

Gold (as sodium gold thiomalate)6-8 g./l.

Arsenic (as a soluble compound)-l0 mg. to 0.12 g./l.

pHl0-12 at a current density of 2-10 amps/ft. and a temperature of 24-82C.

4. The process as claimed in claim 3 wherein the arenic is present inthe proportion of 10-30 mg./l.

References Cited UNITED STATES PATENTS 3,238,112 3/1966 Haslam 20446 XR3,423,295 1/1969 Greenspan 20443 FOREIGN PATENTS 1,027,637 2/1953France.

OTHER REFERENCES Edmund R. TheWs, Metal Finishing, pp. -85, September1951.

Francis C. Frary, Transactions of the Electrochemical Society, vol. 23,pp. 25-97 (1913).

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner U.S. C1.X.R. 204-46

